There are significantly improved required peak rates of a Long Term Evolution-Advanced (LTE-A) system up to 1 Gbps in the downlink and 500 Mbps in the uplink as compared with an LTE system. The LTE-A system is also required to be well compatible with the LTE system. Carrier Aggregation (CA) has been introduced to the LTE-A system to thereby accommodate the improved peak rates, compatibility with the LTE system and full use of spectrum resources as needed.
With carrier aggregation, a user equipment can operate concurrently over a plurality of cells in each of which there are a pair of uplink (UL)/downlink (DL) Component Carriers (CCs) instead of only one set of carriers in the LTE system and earlier wireless communication systems. The respective component carriers in the carrier aggregation-enabled system can be consecutive or inconsecutive, and bandwidths of the respective component carriers may be the same or different; and there is a bandwidth up to 20 MHz of each component carrier for compatibility with the LTE system. The number of cells that can be aggregated for the user equipment is typically up to 5 at present. Moreover the cells with carrier aggregation are further categorized in the LTE-A as follows:
A Primary Cell (PCell), where there is only one of the plurality of cells aggregated for the user equipment to be defined as a PCell selected by a base station and configured to the user equipment in Radio Resource Control (RRC) signaling. A Physical Uplink Control Channel (PUCCH) is configured only in the PCell; and
A Secondary Cell (SCell), where all the other cells than the PCell aggregated for the user equipment are SCells.
In the CA system, the aggregated cells of the UE need to be searched frequently in some cell management procedures (e.g. deletion of a secondary cell, reconfiguration of a secondary cell, activation/deactivation of a secondary cell, reporting of a signal quality of a serving cell, etc.). Cells in an aggregation range each can be identified uniquely by a frequency and a Physical Cell ID (PCI). From the perspective of a signaling overhead, a cell index parameter can be introduced to avoid the two carrier parameters, frequency and PCI, and from being reused frequently. A UE-level cell index parameter encoded in 3 bits is to be introduced as decided in ongoing discussion of the standard, and the index of a cell will be allocated by the network to the UE when the cell is newly added. The cell index of the primary cell is fixed at 0, and the cell indexes of the secondary cells range from 1 to 7,
Due to possibly fluctuating and bursting traffic of the user equipment, that is, a low amount of traffic in some period of time and a high amount of traffic in another period of time, the concept of activation/deactivation has been introduced to the LTE-A system for better conservation of power of the user equipment with a low amount of traffic, where it is specified that the PCell of the UE be prohibited from being deactivated but be kept activated all the time by default whereas the SCells can be activated/deactivated.
For a deactivated carrier, the UE neither needs to detect a Physical Downlink Control Channel (PDCCH)/receive a Physical Downlink Shared Channel (PDSCH) nor needs to transmit a Physical Uplink Shared Channel (PUSCH)/Sounding Reference Signal (SRS)/Channel Quality Indicator (CQI), etc. for a better power conservation effect.
With the LTE-A R11, downlink transmission enhancement has been decided by introducing a novel downlink carrier, referred here as to an downlink extension carrier, for improved spectrum efficiency, a better support of a heterogeneous network (HetNet) scenario and power conservation.
The downlink extension carrier is currently designed in the protocol by supporting at least two synchronous and asynchronous scenarios. In the synchronous scenario, the extension carrier is synchronized in time/frequency with at least one carrier/cell, aggregated for the user equipment, compatible backward with the LTE system, that is, the user equipment does not need to perform an additional downlink synchronization process on the extension carrier. In the asynchronous scenario, the extension carrier is not synchronized in time/frequency with any carrier/cell, aggregated for the user equipment, compatible backward with the LTE system, that is, the user equipment does need to perform an additional downlink synchronization process on the extension carrier.
At present there has not been a relevant mechanism to determine a synchronous cell of an extension carrier despite the defined extension carrier so that the extension carrier is unavailable at the network side and the user equipment.